The present invention relates to optical screens for use in particular with cathode ray tubes of the type in which an electron beam, known as a raster, is rapidly moved over a large number of parallel lines on a display face.
In the above mentioned type of cathode ray tube the display face is coated by a substance known as a phosphor which is excited to emit light by the electron beam. The speed of scanning of the raster and the decay time of the phosphor are chosen to be such that the human eye sees a substantially steady raster display without flickering. The best known use of this type of cathode ray tube is in a television set. However, this type of tube is being increasingly used in aircraft, not only for radar displays but also for displays of aircraft flight and other parameters which are displayed to a pilot.
A problem with cathode ray tubes is that phosphors, by their nature, strongly scatter illumination from external sources, to such an extent that the raster display can become unreadable in high ambient illumination. Anti-reflection coatings are not effective in reducing this scattered light, and in the limited environment of an aircraft cockpit it is difficult or impossible to shield a display face from high ambient illumination during all flight conditions.
The present invention provides an optical screen which enhances the contrast of the image.